The specific aim/objective of this longitudinal, prospective, randomized, non-blinded National Cooperative Study is to test the hypotheses that (1) restriction of dietary protein reduces the rate of progression of chronic renal disease in humans; and (2) dietary protein restriction is nutritionally safe (if effective) in patients with progressive chronic renal failure. Definitive answers will favorably influence the care of chronic renal failure patients worldwide and may reduce the number of patients (and hence the cost) in the ESRD-dialysis-transplant programs. After recruiting patients with the most common renal causes of end stage renal failure (diabetes mellitus excepted) and who match explicit inclusion/exclusion criteria, individualized, adequate "standard diet" and medication schedules, smooth data flow, dietary performance and rate of renal functional progression are established for each patient during an initial control/observation period. Patients are then matched in pairs within diagnostic groups according to level of renal function and rate of progression, and randomized (1) to a continuation of their standard diet or (2) to a reduced protein/phosphate intake with the restriction adjusted to their level of renal function. The methods include critical control of the dietary independent variable by placing a small computer in each participating patient's home in order to record food consumption as it occurs (via an interactive, user-friendly program) and to transmit the information daily to the Center computer. Daily review by project staff will provide timely and accurate feedback, assisted by quantitative urinary collections and calculation of protein catabolic rates from urea or total nitrogen excretions. The dependent variable (renal function), is estimated by reciprocal serum creatinine, creatinine clearance, and GFR = (Curea + Ccr)/2, corrected for completeness of ureine collection by comparison with extrapolated UcrV. Body weight, anthropometrics, serum proteins, and additional monitoring of neuromuscular, neurological and neurobehavioral function are proposed as outcome measures and for patient safety. Psychosocial indices are proposed to address quality-of-life issues. Proposed data analyses are directed to within-patient, control-to-experimental changes, trends and associations among variables that are aggregated within cohorts as defined by the randomization scheme.